This application is based on Korean Patent Application No. 2001-3212 filed in the Korean Industrial Property Office on Jan. 19, 2001, the disclosure of which is incorporated hereinby reference.
1. Field of the Invention
The present invention relates to a positive active material for a rechargeable lithium battery and a method of preparing the same, and more particularly, to a positive active material for a rechargeable lithium battery having improved capability, power, and cycle-life characteristics, and a method of preparing the same.
2. Description of the Related Art
Generally, rechargeable lithium batteries use a material from or into which lithium ions are deintercalated or intercalated for positive and negative active materials. For an electrolyte, an organic solvent or a polymer is used. A rechargeable lithium battery produces electric energy as a result of changes in the chemical potentials of the active materials during the intercalation and deintercalation reactions of the lithium ions.
Such a rechargeable lithium battery, having an average discharge potential of 3.7 V (i.e., a battery having a substantially 4 V average discharge potential) is considered to be an essential element in the digital generation since it is an indispensable energy source for portable digital devices such as a cellular telephone, a notebook computer, a camcorder and so on (i.e., the xe2x80x9c3Cxe2x80x9d devices).
For the negative active material in a rechargeable lithium battery, metallic lithium was used in the early days of development. Recently, however, carbon materials such as amorphous carbon or crystalline carbon-based materials, which reversibly intercalate lithium ions, are extensively used instead of the metallic lithium due to problems of high reactivity toward electrolyte and dendrite formation of the metallic lithium. With the use of carbon-based active materials, the potential safety problems that are associated with the metallic lithium can be prevented while achieving a relatively high energy density as well as a much improved cycle life. In particular, boron may be added to carbonaceous materials to produce boron-coated graphite (BOC) in order to increase the capability of the carbonaceous materials.
For the positive active material in the rechargeable lithium battery, chalcogenide compounds into or from which lithium ions are intercalated or deintercalated are used. Typical examples include LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCoxO2 (0 less than x less than 1), and LiMnO2. Manganese-based positive materials such as LiMn2O4 or LiMnO2 are the easiest to prepare, are less expensive than the other materials, and are environmentally friendly. However, manganese-based materials have a disadvantage of a relatively low capability. LiNiO2 is inexpensive and has high capacity, but is difficult to prepare in the desired structure. LiCoO2 is relatively expensive, but is widely used and is commercially available as it has good electrical conductivity and high cell voltage. Although most of the current commercially available rechargeable lithium batteries (approximately 95%) use LiCoO2 as the positive active material, it is rather expensive. Therefore, there are considerable demands to find a less expensive alternative material.
As one scheme to satisfy such a demand, in U.S. Pat. No. 5,292,601, LixMO2 (wherein M is an element selected from Co, Ni, or Mn, x is 0.5-1) is suggested as an alternative material for LiCoO2 as the positive active material. U.S. Pat. No. 5,075,291 discloses a method of fabricating a rechargeable lithiated intercalation compound for the positive active material including mixing a coating composition comprising boron oxide, boric acid, lithium hydroxide, aluminum oxide, lithium aluminate, lithium metaborate, silicon dioxide, lithium silicate, or mixtures thereof with a lithiated intercalation compound in a particulate form, and fusing the coating compound at a temperature higher than about 400xc2x0 C., thereby coating the particulate with the fused coating compound.
However, there are still further demands and desires for an improved positive active material having enhanced capacity, power, and cycle-life performance for batteries.
It is an object of the present invention to provide a positive active material for a rechargeable lithium battery cell exhibiting better cycle-life characteristics, enhanced power capability, and good voltage performance.
It is another object of the present invention to provide a method of preparing the same.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
In order to achieve the above and other objects, an embodiment of the present invention provides a positive active material for a rechargeable lithium battery including a core including a lithiated compound, and at least two surface-treatment layers formed on the core.
According to another embodiment of the invention, the surface-treatment layers include at least one compound selected from a coating-element-included hydroxide, a coating-element-included oxyhydroxide, a coating-element-included oxycarbonate, or a coating-element-included hydroxycarbonate where the coating element includes at least one element selected from the group consisting of Mg, Al, Co, K, Na, Ca, Si, Ti, Sn, V, Ge, Ga, B, and As.
According to yet another embodiment of the invention, the lithiated compound is one or more compounds selected from the following compounds having formulas (1) to (13):
where 0.95xe2x89xa6xxe2x89xa61.1, 0xe2x89xa6yxe2x89xa60.5, 0xe2x89xa6zxe2x89xa60.5, 0xe2x89xa6axe2x89xa62, M is one selected from the group consisting of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, and rare earth elements, A is selected from the group consisting of O, F, S and P, and X is selected from the group consisting of F, S and P.
According to still another embodiment of the invention, in the positive active material for a rechargeable lithium battery, the surface-treatment layers include one or more coating elements selected from the group consisting of Mg, Al, Co, K, Na, Ca, Si, Ti, Sn, V, Ge, Ga, B, and As.
According to yet still another embodiment of the invention, the positive active material for a rechargeable lithium battery includes a first surface-treatment layer and a second surface-treatment layer, where the first surface-treatment layer includes one or more coating elements selected from the group consisting of Mg, Al, Co, K, Na, Ca, Si, Ti, Sn, V, Ge, Ga, B, and As, and the second surface-treatment layer is formed on the first surface-treatment layer and includes one or more coating elements selected from the group consisting of Mg, Al, Co, K, Na, Ca, Si, Ti, Sn, V, Ge, Ga, B, and As.
According to a further embodiment of the invention, the positive active material for a rechargeable lithium battery includes three or more surface-treatment layers including one or more coating elements selected from the group consisting of Mg, Al, Co, K, Na, Ca, Si, Ti, Sn, V, Ge, Ga, B, and As.
According to a still further embodiment of the invention, a method of preparing a positive active material for a rechargeable lithium battery includes coating a lithiated compound selected from the lithiated compounds represented by the formulas (1) to (13) with a coating liquid including a coating element selected from the group consisting of Mg, Al, Co, K, Na, Ca, Si, Ti, Sn, V, Ge, Ga, B, and As, and drying the coated compound.
According to a yet further embodiment of the invention, the coating and drying includes providing a first surface-treatment layer on the surface of the lithiated compound followed by providing a second surface-treatment layer on the first surface-treatment layer.
According to a yet still further embodiment of the invention, the first surface-treatment layer is formed by first coating the lithiated compound with a coating liquid and first drying the first-coated lithiated compound, and the second surface-treatment layer is formed by second coating the first surface-treatment layer formed on the surface of the lithiated compound with a coating liquid and second drying the second-coated lithiated compound.
According to an additional embodiment of the invention, the coating liquid includes at least one coating element selected from the group consisting of Mg, Al, Co, K, and Na.
According to a yet additional embodiment of the invention, the coating and drying are carried out three or more times by using three or more kinds of coating liquids including at least one coating element selected from the group consisting of Mg, Al, Co, K, Na, Ca, Si, Ti, Sn, V, Ge, Ga, B, and As.